The present invention relates to an ignition coil energizing circuit for use in the internal combustion engine.
FIG. 1 is a circuit diagram showing the conventional ignition device employed in the internal combustion engine. This prior art ignition device includes a signal generator 2 for generating a rotation speed signal which represents the rotation speed of a shaft picked up by a pickup coil of magnet induction type, for example, an ignition timing detector 4 for generating an output pulse representing the ignition timing responsive to an output signal applied from the signal generator 2, a series circuit of a primary winding 6-1 of an ignition coil 6 connected between a power source V.sub.D and the ground, a transistor switching circuit 8 and a resistor 10, and a secondary winding 6-2 of ignition coil 6 and an ignition plug 12 connected in series between the power source V.sub.D and ground. The prior art ignition device further includes a duty cycle control circuit 14 for generating a duty cycle signal responsive to an output signal applied from the ignition timing detector 4. The duty cycle signal serves to determine the period for which the switching circuit 8 is kept ON or OFF, and a circuit 16 for driving the switching circuit 8 responsive to an output signal applied from the duty cycle control circuit 14. A series circuit of resistors 18 and 20 is further connected in parallel with the resistor 10, and an input terminal of a constant current control circuit 22 is connected to the junction between these resistors 18 and 20. This constant current control circuits detects that the current flowing to the primary winding 6-1 of the ignition coil 6 reaches a predetermined value, controls a driver circuit 16 responsive to this current to keep constant the current flowing to the switching circuit 18, and supplies to the duty control circuit 14 an output signal used to achieve duty control in a succeeding cycle.
In the ignition device of this type, when engine failure occurs, the switching circuit 8 may happen to be kept on by the output signal of the driver circuit 16 while the device is rendered inoperative. As a result, a steady-state current flows through the primary winding 6-1 and the transistor switching circuit 8, damaging them due to heat. In order to solve this problem, a circuit may be incorporated in the device in which, after the switching circuit 8 is kept on for a certain period of time, the additional circuit turns off the switching circuit 8. However, when the transistor switching circuit 8 is abruptly turned off, a large current flows through the winding 6-2 and the ignition plug 12 is triggered, reversing the engine rotation and exhausting unburned gas.